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Chronic Wasting Disease

Mule deer (Odocoileus hemionus) are very common in Rocky Mountain National Park. They are smaller than the white-tailed deer and have a black-tipped white tail and white patch on the rump. Male mule deers have forked antlers.

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Chronic wasting disease (CWD) is a contagious neurological disease that affects members of the deer family, causing erratic behavior and weight loss that eventually results in death. CWD is classified as a transmissible spongiform encephalopathy (TSE), a family of diseases that includes bovine wasting disease (mad cow disease) and scrapie, a similar disease that infects sheep. CWD affects members of the cervid family, such as elk, moose, and multiple deer species. Colorado has been one of the epicenters of CWD since its discovery in 1967.

Discovery, Description, and Spread

In 1967 a group of scientists working at the Colorado Division of Wildlife facility in Fort Collins puzzled over why one of the mule deer they had in captivity started behaving strangely. Before long, the deer had died, and the scientists found it to be infected with a strange new disease. Several more infected mule deer were subsequently reported at Colorado facilities. Scientists knew little about CWD until 1978, when Elizabeth Williams, a researcher at the Wyoming State Veterinary Laboratory, classified CWD as a TSE and determined that it was caused by irregularly folded prions—certain proteins—within the deer.

Produced by all mammals, prions are normally used and then broken down within an animal’s cells. When these prions are irregularly folded, the organism’s cells cannot properly break down the protein. When the prions are not broken down, they build up in neural and lymphatic tissue, causing abnormal behavior in the infected animal. This behavior can include a loss of fear of humans, diminished social interaction, increased drinking and urination, and increased salivation. The buildup of prions eventually leads to the animal’s death, the cause of which often appears to be pneumonia. This mistaken diagnosis is believed to be due to the excessive salivation caused by CWD. Infected cervids may show no sign of the infection for up to seventeen months, but when the infection sets in, the average survival time is a few months.

Since 1967, CWD has spread from captive cervid populations in Colorado to free-ranging elk, moose, and deer in many parts of the country. The disease can be spread between cervids via blood, saliva, and feces. Because of this, most of the disease’s spread can be attributed to healthy cervids eating the same plants as infected ones. The first case of the disease reported outside of Colorado occurred at the Sybille Wildlife Research Facility in Wyoming in 1979. Since then, the disease has spread to more than twenty US states and two provinces of Canada.

CWD in Colorado

Significant research on CWD has been conducted at Colorado State University. Since the mad cow disease can mutate to affect humans, many of the studies deal with CWD’s potential to infect noncervid organisms. So far, scientists have infected cows, transgenic mice, and spider monkeys with CWD via cranial injections, showing that the disease can theoretically infect noncervid hosts. Fortunately, the only instances of this infection have been in lab environments. So far, there are no known cases of CWD being transmitted to a noncervid organism in the wild. Studies examining the potential transfer of CWD to humans have shown no conclusive evidence that the disease is a threat to people. Nonetheless, researchers have found that it is possible for the disease to mutate in a way that could infect humans or other animals, with drastic consequences.

CWD has a significant impact on Colorado’s deer and elk population. In areas with dense cervid populations, up to 30 percent of the population can be infected. Consequentially, large numbers are likely dying before they can reproduce. From 2003 to 2013 Colorado’s mule deer population has dropped by 36 percent, despite a reduction in hunting tags. Along with the mule deer, Colorado’s elk herds have started to decline in some parts of the state, despite a lack of predators. CWD is one possible contributing factor to these population decreases.

Colorado Parks and Wildlife (CPW) and its predecessor agencies have attempted to manage the disease, to little avail. The research facility in Fort Collins failed in a first attempt to get rid of the disease in 1985. Then, in 2001, the Division of Parks and Recreation implemented a ten-year plan to prevent the spread of CWD as well as reduce the number of infected animals to 1 percent of the population. Methods included attempting to contain infected herds to the endemic area, killing and removing infected individuals from herds, and closely monitoring cervids being shipped in and out of the state. These efforts were largely ineffective; the disease remains prevalent and continues to spread. Officials continue to confine infected animals to historically infected areas and conduct further research on the disease.

From its discovery at a Fort Collins research facility to its rapid spread across a continent, chronic wasting disease has had a widespread impact on wildlife across Colorado and North America. Wildlife managers and scientists have been baffled, hunters frustrated, and some cervid populations within the state have begun to fall, all because of microscopic, misfolded prions. The history of this disease has only just begun, and Colorado is ground zero.